Electrostatic hand gun cable

ABSTRACT

An improved high voltage cable for usein electrostatic hand guns and includes a semiconductive stranded fiber glass core within a semiconductive shield surrounded by body of insulative material, a flexible conductive sheath in an outer protective covering; the semiconductive core and semiconductive shield provide resistance of not less than 20,000 and not more than 300,000 ohms per foot of cable length.

United States, Patent 119 Smart et a1.

1451 Feb.- 12, 1974 ELECTROSTATIC HAND GUN CABLE [75] Inventors: WilliamL. Sma t; Paul S. Gregg,

- both of lndianapolis, 1nd.

[73] Assignee: Ransburg Corporation, Indianapolis,

Ind.

221' Filed! Apr. 2, 1973 21 AppL No.2 347,240

I [52] US. Cl 338/214, 174/36, 174/105 SC,

. 174/107 [51] Int. Cl...... 1101c 13/00, H0lb 1/06, l-l0lb 9/02 [58]Field of Search 338/214, 208, 299; 174/36,

174/102 R,102 SC, 105 SC, 110 PM, 120

SC, 120 SR, 107, 126 R, 127

[56] References Cited UNITED STATES PATENTS 3,683,309 8/1972 Hirose .i338/214 3,680,027 7/1972 Vitale 338/214 3,644,866 2/1972 D8a1'dllrff....338/214 3,518,606 6 1970 Barker 338/214 x 3,348,186 10/1967 R6861..338/214 3,284,751 11/1966 Barker et a1. 338/214 X 3,167,255 l/l965 Pointet a1. 338/214 X 3,105,219 9/1963 Kozinski 338/214 3,105,218 9/1963Kozinski 338/214 2,790,053 4/1957 Peterson 338/214 X 2,703,356 3/1955Buchanan et a1. 338/214 X FOREIGN PATENTS OR APPLICATIONS 547,481 8/1942Great Britain 338/214 1,490,618 2/1970 Germany 338/214 PrimaryExaminer-Bernard A. Gilheany Assistant Examiner-A. T. Grimley Attorney,Agent, or Firm-Merrill N. Johnson; Harry E. Downer; David H. BadgerABSTRACT An improved high voltage cable for usein electrostatic handguns and includes a semiconductive stranded fiber glass core within asemiconductive shield surrounded by body of insulative material, aflexible conductive sheath in an outer protective covering;-thesemiconductive core and semiconductive shield provide resistance of notless than 20,000 and not more than 300,000 ohms per foot of cablelength.

i 1 Claim, 2 Drawing Figures 1 ELECTROSTATIC HAND GUN CABLE The presentinvention related to a high voltage cable for use in connectingelectrostatic spray coating devices with a separate source of highvoltage.

Electrostatic coating systems frequently use voltages on the order of20,000-100,000 volts of direct current. Voltage of this magnitude placesgreat stress upon electrical insulation that is used to isolate it.Being exposed to such stress, high voltage insulation can failpermitting electrical discharge of the high voltage through the body ofthe insulation or across its surface. In addition, such high voltagescan break down surrounding air and initiate a spark that can travelthrough air to adjacent bodies at lower potential. Electrical isolationof such high voltages is therefore a problem.

Electrostatic coating systems are frequently used with coating materialsthat have a flammable residue or that produce flammable vapors. Sprayingoperations are most frequently conducted in a spray booth to confinesuch flammable materials and to remove them safely from the interior ofan'industrial facility. The Sprayers of electrostatic coating systemsare thus most frequently used. inside a spray booth. The high voltagesource, however, is generally located outside of and at least feet from.the spraying area. The electrostatic charge to. be applied to the sprayparticles must be carried from the high voltage source to theelectrostatic spray device over a distance on the order of 20 feet.

Since the invention of U.S. Pat. No. 3,048,498, hand manipulatedelectrostatic spray guns have been commercially possible. Commercialembodiments of this invention have included electrostatic handguns'having insulating barrels containing resistors with resistances onthe order of 40-1000 megohms to prevent objectionable. electricaldischarges from the spray gun. Such electrostatic hand guns wereconnected by a high voltage cable to the high voltage source outside thespraying area. To permit the electrostatic hand gun to be easilymanipulated by the operator so that he could direct the spray at allsurfaces of complexly-shaped articles, the high voltage cablev had to beflexible. The achievement of flexibility, however, was not alwaysconsistent with the requirement that the electrical insulation of thehigh voltage cable withstand the tendency of high voltage to causeinsulation failure. I

In such commercial electrostatic hand gun systems, coaxial high voltagecables were used. The high voltage conductor was a metallic wire at thecenter of the high voltage cable. Surrounding the wire was a body ofinsulating material, generally a polyethylene. Surrounding thepolyethylene insulation was the grounded metallic braid to act as abarrier to any electric discharge which might occur in the event of aninsulation failure of the dielectric material, and to prevent such anelectric discharge from occurring to personnel who may be using theelectrostatic hand gun. For convenience, a protective covering of rubberor plastic was provided as the outermost covering of the cable.

Many attempts were made to improve upon such high voltage cables.Several attempts were made to incorporate into the high voltageconductor at the center of a coaxial high voltage cable, the entireresistance that was otherwise used within the barrel of theelectrostatic hand gun. In essence, this approach was an attempt tostretch the high resistance from within the barrel throughout the lengthof the cable extending to high voltage source. In addition toeliminating a high voltage connection between the resistor and the highvoltage cable within the electrostatic spray gun itself, this approachhad the advantage of limiting the energy available in any electricdischarge occurring in the high voltage cable because of the breakdownof the surrounding dielectric material. British Patent Specification1,051,184 and U.S. Pat. No. 3,167,255 both disclose such an approach.Because of the technical difficulties in manufacturing such a cable,however, it never became a commercial reality. In addition, high voltagecables for electrostatic systems were manufactured having a centralconductor made of a plurality of shortb'ut rigid high resistancesegments that extended throughout the length of the cable (see U.S. Pat.No. 3,348,186). Cables of this latter type thus included many highvoltage electrical connections at the junctions of the short highresistance segments. Furthermore, such cables included necessarily, aplurality of high voltage conductor surface discontinuities at theseconnections at which nonuniformly high potential gradients could occur.This plurality of connections of surface discontinuities was undesirableand accentuated the tendency of high voltage to break down theelectricalinsulation of the cable.

Our invention is an electrical cable used to connect an electrostatichand spray gun to a separate high voltage source. Such a cable isgenerally on the order of at least 20 feet, although somewhat longer andshorter lengths of cable may be used. It includes a high voltageconductor which is smooth throughout its length and has a uniformdistributed resistance of at least 0.5 to about 7.5 megohms equallydistributed along its length. Preferably, the cable is thirty feet inlength and has a total resistance of five megohms, or about 167,000 ohmsper foot of cable length. The cable is small, having an overall diameterof 7/16 of an inch. This cable solves the difficulties of providing acable that can be reliably manufactured for commercial use and providesthe flexibility needed for manipulatable hand guns, with the ability towithstand the tendency of high voltage to break down its electricalinsulation.

FIG. 1 is a diagrammatic sketch of an electrostatic hand gun systemincluding such a cable.

FIG. 2 is a cross-sectional view of the cable.

FIG. 1 shows an electrostatic hand gun in use in a spraying area.Electrostatic hand gun 10 is connected to the high voltage source 11 by25 feet of high voltage cable 12. One such electrostatic hand gunincludes within its barrel portion a high voltage resistor 13 having aresistance on the order of megohms. The high voltage for such a hand gunproduces 60 kilovolts of direct current.

The high voltage cable 12 which carries electrostatic charge from thehigh voltage source to the hand gun as shown in FIG. 2, includes as thehigh voltage conductor 14 stranded conductive fiber glass. The centralconductor is comprised of many fibers each having a very small diameter.When stranded, they produce a central high voltage conductor having adiameter of 0.015 inches. This stranded fiber glass conductor is treatedto provide a resistance on the order of 20,000 to 300,000 ohms per foot.Immediately surrounding the stranded fiber glass conductor is a coronashield 15 of conductive polyethylene, consisting of a homogeneousmixture of polyethylene with a small percentage of carbon black. Theconductive polyethylene minimizes variations of potential gradient whichmight occur at the surface of the high voltage conductor because of gastrapped during the manufacturing process or surface discontinuities. Atthe boundary between the corona shield and the surrounding insulation,the corona shield is typically 0.050 inches in diameter. The combinedresistance of the semiconductive stranded fiber glass conductor and thesemiconductive corona shield should preferably measure not less than20,000 nor more than 300,000 ohms per foot of cable length.

surrounding the corona shield is the high voltage cable insulation 16.The insulation is a high grade polyethylene that isolates the highvoltage charge at its center. Such a polyethylene is typically DFD 60005NT (Natural), manufactured by Union Carbide. Surrounding the insulationis a conductive braid 17. Such a braid is comprised of 160 strands of30-gauge copper alloy wire braided together around the insulation.Surrounding the braid itself is a sheath 18 typically 0.030 of an inchthick polyurethane. Total diameter of such a cable structure can be 7/16inch.

Because of the improved conductive fiber glass high voltage conductor,this cable can be easily manufactured. The fiber glass center conductorprovides strength and flexibility needed to manufacture the cable.Furthermore, its smooth outer surface is relatively free of surfaceimperfections, which can accentuate the tendency of the high voltage tobreak down electrical insulation. The fiber glass provides a chemicallyand dimensionally stable medium and permits a conductivity which can becontrolled within the limits needed for a comercially satisfactoryproduct. Sufficient resistance can be incorporated into the centralconductor so that in the event of an insulation failure, the energystored throughout the cable by the high voltage electric charge presenton the high voltage conductor is dissipated by the resistance of thehigh voltage conductor as the electric charge travels down the cable tothe point of the insulation failure. Thus, the energy available in anelectrical discharge between the high voltage conductor and theconductive braid may be limited by the action of the resistance of thehigh voltage conductor. Because of the reliability in achieving stablehigh resistance values to the high voltage conductor, it is possible toobtain a cable where the energy available in the event of an insulationfailure is below the level necessary to ignite most combustible vaporsand residues encountered in spray finishing operations. Incorporation ofsuch a cable into an electrostatic hand gun system thus improves thesafety of the system.

The specific cable described above is typical of those used inelectrostatic coating systems. However, this invention is capable ofincorporation into cables other than that specifically described above.

We claim:

1. An electrical cable used to connect an electrostatic spray gun to aseparate high voltage source, said cable having a stranded fiber glasscore within a semiconductive shield surrounded by a body of insulativematerial, a flexible conductive sheath in an outer protective covering;the improvement which consists of the core and the shield togetherhaving a resistance of not less than 20,000 and not more than 300,000ohms perfoot of cable length the improvement which consists of the corebeing semiconductive and with semiconductive shield providing aresistance of not less than 20,000 and not more than 300,000 ohms perfoot of cable length.

. j UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,792,409 Dated February 1.2, 1.974

Inventor(s) WILLIAM L. SMART, ET. AL.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

In the Abstract, line 1, "usein" should read use in Column 3, line 11,"surrounding" should read Surrounding Column 4, claim 1 should read asfollows: I i Q 1. I'n an'e'lectrical cable'used to connect anelectrostatic spray gun to a separate high voltage source, said cablehaving a stranded fiber glass core within a semiconductive 7 hieldsurrounded by a body of insulative material, a flexible conductivesheath in an outer protective covering; the improvement which consistsof the core and-,gij the shield together having a resistance of not lessthan 20,000 and not more than 300,000 ohms per foot of cable length.

Signed and sealed this 13th day of August l974.

(SEAL) Attest: I

MCCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents R P0405, (W69) uscoM -oc wan-Poo 1 "-5. GOVERNMENT PRINTINGOFFICE: IQ, "'l .:.'l"'

. PUNTTED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.792,409 Dated February 12, 1974 Invent-or(s) WILLIAM L. SMART, ET. AL.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

In the Abstract, line 1, "usein" should read use in Column 3, line ll,"surrounding" should read Surrounding Column 4, claim 1 should read asfollows: I

-- l. In an electrical cable used to connect an electrostatic spray gunto a separate high voltage source, said cable having a stranded fiberglass core within a semiconductive shield surrounded bya body ofinsulative material, a flexible conductive sheath in w an outerprotective covering; the improvement which consists of the core andgjgthe shield together having a resistance of not less than 20,000 and notmore than 300,000 ohms per foot of cable length.

Signed and sealed this 13th day of August 1974.

(SEAL) Attest: I

McCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents FQRM P0405) USCOMM-DC wan-Pen ".5, GOVERNMENT PRINTING OFFICEl", Q""gl",

1. An electrical cable used to connect an electrostatic spray gun to aseparate high voltage source, said cable having a stranded fiber glasscore within a semiconductive shield surrounded by a body of insulativematerial, a flexible conductive sheath in an outer protective covering;the improvement which consists of the core and the shield togetherhaving a resistance of not less than 20,000 and not more than 300,000ohms per foot of cable length the improvement which consists of the corebeing semiconductive and with semiconductive shield providing aresistance of not less than 20,000 and not more than 300,000 ohms perfoot of cable length.